Arduino Ultrasonic Sensor (HC-SR04) Distance Code

hardSensorsArduino (C++)

The HC-SR04 sends a sound pulse and times the echo — from that time we calculate the distance.

The question

An HC-SR04 has TRIG on pin 11 and ECHO on pin 3. Measure the distance in centimetres (distance = duration × 0.034 / 2) and print it to the Serial Monitor at 9600 baud every 200 ms.

The sketch

sketch.ino

void setup() {
  Serial.begin(9600);
  pinMode(11, OUTPUT);
  pinMode(3, INPUT);
}

void loop() {
  digitalWrite(11, LOW);
  delayMicroseconds(2);
  digitalWrite(11, HIGH);
  delayMicroseconds(10);
  digitalWrite(11, LOW);
  long duration = pulseIn(3, HIGH);
  float cm = duration * 0.034 / 2;
  Serial.print("Distance: ");
  Serial.print(cm);
  Serial.println(" cm");
  delay(200);
}

Open the Arduino simulator »

Wiring

HC-SR04: VCC → 5V, GND → GND, TRIG → pin 11, ECHO → pin 3.

How it works

1A 10-microsecond HIGH pulse on TRIG starts one measurement.
2pulseIn(ECHO, HIGH) returns how long the echo took, in microseconds.
3Sound travels about 0.034 cm per microsecond; the pulse goes there and back, so divide by 2.

Common mistakes

Setting ECHO as OUTPUT — it must be INPUT.
Forgetting to divide by 2, which doubles every distance.

Now solve a similar question yourself

A new Arduino question every time, checked instantly.

Practice questions